In order to derive a boosting function from a servo unit equipped with a hydraulic braking unit, the control pressure of the servo unit has to be maintained to a desired value. For the purpose of maintaining the control pressure, the pressure of an accumulator that accumulates the control pressure of the servo unit is monitored. If the level of the monitored pressure of the brake fluid stored in the accumulator fails to reach the desired value, the motor is turned on to drive the pump that sucks and discharges the brake fluid from a master reservoir so as to be supplied to the accumulator.
The control of the power supply to the motor for adjusting the control pressure of the servo unit is executed through a relay circuit. The relay circuit is formed of a parallel relay circuit (fail-safe relay) in which two or more relays are connected in parallel in view of a fail-safe (redundancy) function.
In the case where the aforementioned parallel relay circuit is disconnected, the motor cannot be turned on, thus failing to derive the boosting function from the servo unit. It is, therefore, necessary to detect the disconnection of the relay circuit. Japanese Patent Publication Laid-Open No. 2000-108887 discloses an art for detecting disconnection of the parallel relay circuit based on a time period taken for charging the capacitor provided in the power supply line for the relays of the parallel relay circuit.
The art as disclosed in Japanese Patent Publication Laid-Open No. 2000-108887, however, fails to detect the disconnection that occurs in a part of relays of the parallel relay circuit. In case of the aforementioned partially disconnected relay circuit, it is possible to use the relay other than the disconnected one in the parallel relay circuit to drive the motor. In the aforementioned case, current is applied to the other relay for an elongated period of time, resulting in increased heat generation amount. Therefore, the disconnection that occurs in the part of the relays in the parallel relay circuit is desired to be detected.